Means fob cable-sheath bonding



July 16, 1929. H. HALPERIN ET AL MEANS FOR`CABLE SHEATH BONDING FiledOct. 6, 1927 VPatented July 16, 1929.

UNITED STATES PATENT OFFICE.

HERMAN HALPRINAND KENNETH W. 'MILLEIL OE CHICAGO, ILIINOIS.

MEANS' Fon. CABLE-SHEATH l BoNnING.

Application filed October 6, 1927. Serial No. 224,339.

. ground.cables used in three phase transmission work. v

It was formerly the practice in the underground three-phase transmissionofalternating currents to place the three conductors in a single cablebut as the values of the voltages and currents increase, in accordancewith modern practice, this often becomes impract-icable. This resultedinthe adoption y of the present method in which the conductors are placedin separate cables, which are placed in suitable conduits under ground.These cables are arranged substantially parallel and, of necessity, arenot placed very f far apart. In this arrangement several very seriousobjections arise, due to induced sheath voltages or currents. 4

When alternating currents flow inthe conductors of single conductorcables, there is induced a voltage in each sheath, and this voltage isdirectly proportional to the current and the length of each section ofcable between the manholes. If the sheaths of the three cables areconnected by the ordinary method, which is solid bonding in eachmanhole, then this induced voltage causes a large current to flow inthesheath, with resultant heat losses that considerably reduce the cur`rent carrying capacity of the cable; for instance, about twenty per centon a' singleconductor (i6-k. v. cable. Tlf, however, the sheath lengthsare made discontinuous by insulating joints and are specially connectedby bonds or impedanees, current may be prevent-ed in the sheaths orgreatly restricted, and the sheath losses practically eliminated. Inthis case the induced sheath voltages are not consumed in producing'sheath current and are therefore presentl on the sheath.-

Various methods have been vproposed for 4connecting vthe discontinuouslengths of sheaths. some of which will be discussed; None of thesemethods haveproved to be entirely satisfactory, all being open toseveral serious objections. Y.

One of the methods referred to is known as the cross-bonding method.This is, in

effect, simply sheath transposition. -In-practice it has severaldisadvantages. It does not lend itself readily to irregular conduitlengths, such as exist particularly around 5i stations. Sheathy voltagesto ground at the intermediate portions of a three-length unit are thefull amount which is induced in one sheath. During short circuits, theseinduced voltages: increase v1n Arent, and may reac roportionv to thecurlarge values. Also serious difficult-ies ma be introduced into thelocation of faultsy y electrical methods, the electrical signal beingtransposed between cables and carried by the fault.

A second proposed method with which we are familiar employs single-phaseiron-core reactors in series with the sheaths of singlelconductorcables. In thiscasethe circuit of the iron core is not closed. This isopen to o the objection that, during' short circuits, when the currentflowing would be several times normal, the voltage across the reactorwould be correspondingly higher.

In a third method, which is an improvement of the second method abovediscussed, a single-phase iron-core reactor is` connected in series withthe sheaths of single-conductor cables and is grounded. In this thirdmethod. however, the core of the reactor is designed with a closed ironcircuit so proportioned as to approach magnetic saturation under normalconditions. As a rev sult, during flow of abnormal currents such' asoccur .during short circuits, the core'becomes saturated permitting alargeI amount of current to flow through the reactor thus limiting thevoltage across the reactor, this cable length. This reduces byat leastfifty 10 per cent the danger of trouble from A. C. electrolysiseli'ects.

'of' providing a separate vreactor for each phase, and the 'coils ofthese reactors are ratherglarge and expensive, both' ofl which factorsadd very materially to the cost of this method. We have found that theabove noted objections can be avoided by using `a single reactor havingseveral coils properly connected to the sheaths of -the cab1es gto actin flux opposition. This practically eliminates the flow of inducedsheath currentsl during normal operation, and prevents abnormally highsheath voltages during cable failures. By using a single reactor thecost -of'installatiom as well as the cost of the reactors used, isgreatly reduced as comared to the third,A method above discussed. urtherobjects and advantages of our inventionfwill appear from the detaildescription.

Inthe drawings y Fig. 1 isa diagrammatic view of our invention asapplied showing the reactor .coils connected in vseries with the sheathsofthe respective cables;

Fig. 2-isa diagrammatic viewv of our in-I vention as applied showing thereactor coils connected to. the. cable sheaths, the latter beingtransposed; F

' Fig. 3 is asemi-diagrammatic view' of the reacto` used inI the methodof Fig. 1;

Fig. 4 is a semi-diagrammatic View of the reactor used int-he method -ofFig.l2.

In the method illustrated in Fig. 1, each of the cablesl 'is providedwith a sheath 2 usually of lead or lead alloy. This sheath -isy dividedinto sections or lengths by insulated joints 3, of any known type, the

joints interrupting the electrical continuity of the sheath. Preferably,each sheath section extends fromfone man-hole to another, the insulatedjoints being located in the {nan-holes as is customary, though thesey.joints 'may be located at other points also,

if required.

.f A reactor' such as that illustrated in Fig. 3 )i's used in vthemethod of Fig'. 1. This reactor has iron core members 5, one core foreach phase. These cores are joined and magnetically closed at each endby the members 4, which permit flux interlinkagesbetween the phases inany combination. Coils 6 are mounted about the -respective cores. Eachof these coilsma'y be grounded at :its

' phase.

mamas centreat 7 by a lwire 8, the same wire be' ing used for groundingall of the coils.

similar portions or sections. The three coils are all similarly wound sothat the magnetlc fluxes generated thereby in the core member 4: act inopposition when the currents are equal in phase and magnitude asindicated by the arrows 11 and 12, respectively, in Fig. 3.- They coremember 4 is also connected to wire 8, as at 9, so as to be groundedthereby. As shown in Fig. 1, the coils 6 bridge the joints 3 and areconnected in series with the sheath lengths of 'the respective cables,these coils being grounded at their midpoints as noted.

During a failure on a line, in cases Where\ a reactor of usual type isused, in addition to the voltages across the reactor on account of theinduced voltage in its adjacent i Coil 6 is thus, in effect, separatedinto two length ofA cable,there would be an additional voltage drop onaccount of the fault current flowing back over the sheaths and throughthe reactors' to the stations `at `the ends of the faulted line. Forexample, `on a 66 k. v., three phase, single-conductor, undergroundline, during a short circuit there would be a currentvof say 2,000amperes flowing .in the conductor ofthe faulted entirely over the three4'cables of the given Y line, which wouldbe practically the case in manyinstances, then the sheath of each' cable 'would be carrying about onethirdof the current.4 This returning fault current, together witlnthevoltages'induced by the current" in the-copper, -would cause anobjectionably large voltage drop `ac ross single phase reactor coils,for with single phase reactor coils this voltage is vlimited only byVflux saturation i n the cores. Vith our invention ,this objection visavoided since the .returning faultl current flowing back over thesheaths of the three cables would be`l nearly equal in amount andphaseon each sheath, and in the same direction through all vof the coilso'f the three-coil reactor. This results in magnetic linx opposition inthe iron core, as previously explained, so there would be practicallyno'voltage drop` in the coils due to this returning fault current. l

Furthermore when an iron core is oper- If this 2,000 amperes wouldreturn' ated at and above the saturation point, the

magnitude of the triple voltage harmonic becomes large resulting, insome instances,

in a voltage increase as high as forty-percent from this factor alone.With our three-coil reactor, as is well `known from three-phasetransformer'operatiom triple-and higher harmonies can not appear duringnormal operation, w`hich is a decided advantage. Introduction of tripleand' higher voltage 'and current harmonics. on power cable sheaths maycause interference on telephone circuits in adjacent duct structures dueto the well known additive effects of these harmonics which are in phasefor all three phases of the power circuit.

. Also, during parallel flow of return failure current through the threecoils of our reactor, the triple and higher harmonic volt-.- ages willbe much smaller than in cases where a reactor which` does not have themagnetic fluxes of its coils opposed is used. This is because, in ourreactor, flux opposition exists for the largest portion of the returnfault currents which are practically equally divided in the three coils,Only a small -residual fiux is produced in the core due to theinequality of division of the returning fault currents, and saturationof4 the iron core is' absent or much reduced. As a result of thecombination -of the effects of fiux lopposition on the (iO-cycle andtriple and higher harmonic voltages during failures, the maximum voltagedifference across our three-coil reactor for a 2000 ampere fault currentwould be but about thirty volts.

The corresponding drop across a reactor of a type not having the fluxesof the coils acting in opposition would be fifty-five volts or higher. l

It is an accepted law that 4currents and fluxes will always adjustthemselves so that the total impedance or voltage drop across anyelectrical net-work will be a minimum. With linx linkages providedbetween the phases in our three coil reactor there is a greater freedomof adjustment vso that our three-coil .reactor can never give lessprotection than reactors of the other types above discussed, under anyv('omblination of short circuit lcurrents in the cable conductors, orreturning failure currents from the same or anyother circuit. in generaldue to greater flexibility of current adjustment it will give muchbetter protection. l

In the method illustrated in k2. We use a three coil reactor'su'chl asthat illus,-

trated in Fig. `4.. This reactor is similar-to the reactor Lof Fig. 3.with the exception that the three coils 10 are continuous and 'A 'aregrounded at their ends instead of at their 50 centers. These coils areconnected in Y or st-ar at their other ends to the lengths of sheaths ofthe respective cables, these sheath lengths being transposed bv bondingas illustrated. I'nvthis method the normal sheath voltages Ito groundare about fifteen per cent,J

higher than with the straightseries connection shown in Fig. 1. It hasthe advantage, however, of the minimum voltage drop along thesheaths'during the flow of return failure currents. When used with thisconnection, the principal advantages of our three-coil reactor are, itsincreased-abilitv to pass fault current to ground due to the Huxopposition effect. and its reduced size and cost.

In both methods We use'but one three coil ltion the current, voltage and'magnetic flux vectors are not in opposition but rather in the normalthree-'phase relation so that the efiiciency of preventing sheathcurrents under normal conditions is in no way impaired. The saturationor lack of saturation ofthe iron core under normal operating conditionsis not essential. The core m y, therefore, be so constructed andproportioned that it approaches magnetic saturation or not, as'desired.Under ordinary conditions, however, We prefer that the 'core does notapproach magnetic saturation under normal operating conditions. i

It is to be noted that any ldirect current flowing through the sheaths,due to stray railwaycurrents or any other causes, within practicallimits will not" saturate .the core,

because of thejfact that these currents will divide nearly'equally inthe several cable sheaths andA reactor coilsand produce opposingmagneto-motive forces lwithin the iron Whatweclaim is: .p A

1. In combination with the cable sheaths of a three-phase transmissionsystem, a reactor includinga core of magnetic materialv and three coilsmounted von the core and relatively disposed to cause their magneticfluxes to act in opposition, the sheaths havl and `being divided intolengthsfthe coils bridging the gaps between adjacent ends of the sheathlengthsof the respective cables.

2. In combination with the cable sheaths of a three phase transmissionsystem, a reactor including a core of magnetic material and three' coilsmounted on the core and relativelyv disposed to cause their magneticfiuxes to act in opposition, the sheathshaving their electricalcontinuity interruptedv andjbeing divided' into lengths, vthe coilsbridging the gaps between adjacent ends of the sheath lengths of therespective cables, and means electrically connecting the coils attheir'midpoints and grounding them.

3. In combination with the cable sheaths of a three-'phase transmissionsystem, a reactor including a core of magnetic material having threecoil receiving. elementsy and coils mounted about the respectiveelements of the core andn magnetically connected 'ing their electricalcontinuity interrupted actor including a core of magnetic materialvhaving three coil receiving elements and similarly directed coilsmounted about the respective elements of the core, said coilsbeing\similarly connected across the gaps between Athe sheath lengths ofthe respective cables and grounded.

5.- In combination with the cable sheath of a three-phase transmissionsystem, a reactor including a core of magnetic material having threecoil receiving elements and similarly disposed and wound coils mountedon said elements, said coils being similarly con-i nected across thegaps between the sheath leigths of the respective cables and grounde o6. In combination with the cable sheaths ol' a three phase transmissionsystem, a reactor including a core of magnetic material `having threecoil receiving `elements' and similarly disposed and wound coil: mountedon said elements, said coils being similarly connected to the sheathlengths ot the respective cables and groundedand the conu nectionsbetween the sheath lengths being transposed.

A, eq

7. In. combination `with the cable sheaths of a three phase transmissionsystem, a reactor including a core of magnetic material havlng;three,co1l receiving elements, and

coils mounted on said elements, each of the sheaths being'electicallydiscontinuous, said coils being connected across the gaps in therespective sheaths, the three core elements being magnetically connectedin'three phase 'arrangement to prevent4 the How of triple harmoniccurrent inthe coils and sheath circuits.

8. In combination with the cable sheaths the coil and sheath circuits.

9. In a multiphase power transmission system wherein separate sheathedcables are used foreachv phase, the combination. of a' reactor includinga .core of magnetic material having a coil receiving element `for eachcable of the transmission system, and wound coils mounted on said coreelements, said sheaths being electrically discontinuf mancia ous andsaid coils beingy connected to bridge the gaps between the sheathlengths, saidy l -coils being grounded at their midpoint thus 'sionsystemwherein the respective conductros are enclosed in separate\metallic sheaths each comprising a number of separate sectionsinsulated from one another, and wherein the sections for the respectiveconductors are cross-connected to rovide vcircuits each comprisingsheath sections of the three several conductors, in which circuits theinduced voltagesare in op osed directions at different sections, the comination of yastar connected impedance connectedyto the respectivesheaths, the neutral point of the impedance being grounded.

l1. An electric circuitf for transmitting al ternating current wherein,the conductors of the circuit are enclosed in separate metallicsheaths, and wherein the sheaths are in sections insulated fromv oneanother and the sect-ions for one conductor are cross-con-V nected withthose of another conductor so as to provide sheath circuits in which theinduced voltages are in ,opposite directions f at different sections,ycharacterized by. the

fact that thereis combined with the lsheath sections of the respectiveconductorsv a plurality of groundedreactance coils mounted on a commonmagnetic core structure.v I

12. An .electric cablesystem for ,transf mitting three phase. currentswherein three insulated conductors respectivelyenclosed in metallicsheaths physically separated from each other constitute the transmisslonline', the sheaths of the several conductors being in sections insulatedfrom each other .and the sections of. the three conductors, beingserially connected injtransposed rela-y tion so that'the inducedvoltages inl the serially connected sections of sheaths act'toopposeeach other and' toprevent the iow of current,` characterized bythe fact/thatthere is connected to the. respective sheaths at aplurality of spaced intervals grounded'magnetic coils, the coils at eachof the respective points having a commonvlmagnetic circuit.

13. An electric .circuit for. transmitting three phase current,comprising threeinsulated conductors respectively enclosed in metallicsheaths physically separated from eachother, the sheaths of .the severalconductors beingin sections insulated from each other, and groundedreactance coils :icone- -necting adjacent sheath sections ofeachconductor, the coils connecting the sheath sections, of the dierent,phase conductors bemg 1,721,018 l A J5 mounted on a common magnetic coreso as' netically interlinking the respective sheath to produce magnetomotive forces in opposections of al1 of the phase conductors to-1 10sition. p gether.

14. A poly-phase power transmission sysn witness whereof, We hereuntosubscribe tem comprising a separate metallic sheathed our names this 3rdday of October, 1927.

cable for each phase, 'each of the cablesheaths comprising a number ofseparate in- HERMAN HALPERIN.

sulated sections, and means for electromag- K. W. MILLER.

